TaskSource and TaskTarget carry disk_id on the wire, but the execution
plan map built for the admin UI dropped the field entirely. On a
multi-disk node holding shards of the same volume, there was no way to
tell from the plan which disk would receive each shard. Include
disk_id on each endpoint and target_disk_id on each shard assignment,
and extend the existing execution-plan test to set and assert the
field.
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